Slurry monitoring system and method therefor

ABSTRACT

A slurry monitoring system ( 20 ) is provided. A filter ( 34 ) encompasses a filter chamber ( 36 ) containing a purge piston ( 66 ). During a monitor operation ( 100 ), the purge piston ( 66 ) is retracted, and slurry ( 32 ) enters the filter chamber ( 36 ) through the filter ( 34 ), then passes into an outlet pipe ( 60 ), where a characteristic of the slurry ( 32 ) is monitored by a monitoring device ( 62 ). During a purge operation ( 102 ), a flush valve ( 54 ) is opened to allow water ( 56 ) to pass into the filter chamber ( 36 ) through an inlet pipe ( 52 ). The purge piston ( 66 ) is extended to force filtered slurry ( 110 ) from the filter chamber ( 36 ). The water ( 56 ) dilutes filtered slurry ( 110 ) remaining in the filter chamber ( 36 ). The flush valve ( 54 ) is then close and the purge piston ( 66 ) retracted. This forces diluted slurry ( 140 ) to pass out of the filter ( 34 ). This purges the filter ( 34 ). Diluted slurry ( 140 ) also passes into the outlet pipe ( 60 ), which purges the outlet pipe ( 60 ) and monitoring device ( 62 ).

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to the field of slurry monitors.More specifically, the present invention relates to the field of slurrymonitors having self-purging capabilities.

BACKGROUND OF THE INVENTION

[0002] Typical applications for slurries are the scrubbing of facilityexhausts. For example, the exhaust of a coal-fired facility wouldtypically contain acidic flue gasses and fly ash. Both poseenvironmental problems.

[0003] The acidic flue gasses are predominantly sulfur dioxide (SO₂) andhydrogen chloride (HCl). These acidic flue gasses combine with moisturein the atmosphere to produce sulfuric and hydrochloric acids, which thenprecipitate as acid rain.

[0004] Fly ash is the finely divided residue that results from thecombustion of ground or powdered coal. Fly ash is a pozzolan, i.e., asiliceous material which, in the presence of water, will chemicallycombine with calcium oxide (lime) to produce a cementitious material. Ifallowed to pass into the atmosphere, fly ash would precipitate, eitherdirectly or in combination with atmospheric moisture, and form cloggingfilms, scums, and sediments.

[0005] To inhibit the environmental consequences of such an exhaust, itis desirable to remove acidic flue gasses and fly ash from the exhaustbefore releasing the exhaust into the atmosphere. This is done in ascrubbing process, typically with the use of a lime-based slurry.

[0006] Sulfur dioxide reacts with lime to form calcium sulfite, whichcan then be air-oxidized to form calcium sulfate dihydrate (gypsum):

[0007] SO₂+Ca(OH)₂→2CaSO₃+H₂O

[0008] 2CaSO₃+O₂→2CaSO₄

[0009] CaSO₄+2H₂O→CASO₄.2H₂O

[0010] Similarly, hydrogen chloride reacts with lime to form calciumchloride:

[0011] 2HCl+Ca(OH)₂→CaCl₂+2H₂O

[0012] Fly ash combines with the lime in the slurry to form siliceouscompounds having cementitious properties. Being cementitious, thesecompounds tend to agglutinate into masses within the slurry.

[0013] While removing the acidic gasses and fly ash from the exhaust,the scrubbing process contaminates the slurry. The byproducts of thescrubbing process, calcium sulfite, calcium sulfate dihydrate, calciumchloride, and various siliceous compounds, enter and become a part ofthe slurry. These byproducts may be removed from the slurry by variouschemical and/or separation techniques known to those skilled in the art.

[0014] The slurry is typically recycled and refreshed for bothenvironmental and fiscal considerations. This requires thatcharacteristics of the slurry be adjusted appropriately.

[0015] There is, therefore, a need to control the characteristics of alime-based slurry used in the scrubbing of the exhaust of a coal-firedfacility. This is often accomplished by monitoring a desiredcharacteristic either periodically or substantially continuously, andadjusting the slurry in response to this monitoring. For example, if thedesired characteristic is slurry density and if monitoring indicates theslurry is too dense, then more water may be added. Similarly, if themonitoring indicates the slurry is insufficiently dense, then more limemay be added. Such adjustments, in conjunction with contaminant removal,allow a slurry to be used indefinitely, with minimal wastage of both thewater and the lime.

[0016] In scrubbing the exhaust of a coal-fired facility, the acidicflue gasses and fly ash pass into the slurry. This results in a slurrythat is acidic, abrasive, and lumpy, with some “lumps” (agglutinationsof siliceous compounds from fly ash) attaining a significant size andmass.

[0017] The contaminated slurry attacks the structure used to contain it,especially when the slurry is in motion. The walls and piping containingthe slurry are continuously eroded, abraded, and pounded by the slurry.This results in further contamination of the slurry by components of thecontainment structure. A typical slurry therefore may not only containwater and lime; but also sulfur dioxide, hydrogen chloride (hydrochloricacid), and fly ash directly from the exhaust; sulfuric acid, calciumsulfite, calcium sulfate dihydrate (gypsum), calcium chloride, andagglutinations of cementitious siliceous compounds as exhaustbyproducts; and even metal fragments, fiberglass, pieces of concrete,rebar, and chevrons from the containment structure itself. This providesan extremely hostile environment into which to place a monitoring device(e.g., a density meter). Filters, filter housings, small-bore piping,and monitoring devices used in such an environment are often clogged,damaged, or destroyed by the contaminated slurry.

SUMMARY OF THE INVENTION

[0018] Accordingly, it is an advantage of the present invention that aslurry monitoring system and method therefor is provided.

[0019] It is another advantage of the present invention that a slurrymonitoring system is provided that incorporates both a monitor operationand a purge operation.

[0020] It is another advantage of the present invention that a slurrymonitoring system is provided that, during a monitor operation, monitorsa slurry characteristic through a filter chamber encompassed by aprotected filter.

[0021] It is another advantage of the present invention that a slurrymonitoring system is provided that, during a purge operation, purges afilter chamber and an encompassing filter.

[0022] It is another advantage of the present invention that a slurrymonitoring system is provided that, during a purge operation, purges amonitoring device.

[0023] The above and other advantages of the present invention arecarried out in one form by a slurry monitoring system configured tomonitor a characteristic of a slurry. The system incorporates a baseplate, a filter coupled to the base plate, a filter chamber encompassedby the filter and configured to contain a filtered slurry, a purgepiston contained within the filter chamber, an outlet pipe coupled tothe filter chamber and configured to pass the filtered slurry, and amonitoring device coupled into the outlet pipe and configured to monitorthe desired characteristic of the filtered slurry.

[0024] The above and other advantages of the present invention arecarried out in another form by a slurry monitoring method incorporatingthe activities of causing a slurry to flow through a slurry flow chamberinto which a filter encompassing a filter chamber has been placed,maintaining a purge piston in a first position proximate a first end ofthe filter chamber for a first predetermined length of time to effect amonitor operation, extending the purge piston from the first position toa second position proximate a second end of the filter chamber to begina purge operation, retaining the purge piston in the second position fora second predetermined length of time to effect the purge operation, andretracting the purge piston from the second position to the firstposition to terminate the purge operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] A more complete understanding of the present invention may bederived by referring to the detailed description and claims whenconsidered in connection with the Figures, wherein like referencenumbers refer to similar items throughout the Figures, and:

[0026]FIG. 1 shows a schematic view of a slurry monitoring system inaccordance with a preferred embodiment of the present invention;

[0027]FIG. 2 shows a side view of a filtration unit for the system inFIG. 1 in accordance with a preferred embodiment of the presentinvention;

[0028]FIG. 3 shows an end view of the filtration unit of FIG. 2 inaccordance with a preferred embodiment of the present invention;

[0029]FIG. 4 shows a cross-sectional view of a purge piston for thefiltration unit of FIG. 2 in accordance with a preferred embodiment ofthe present invention;

[0030]FIG. 5 shows a flow chart of a slurry monitoring process for usewith the system of FIG. 1 in accordance with a preferred embodiment ofthe present invention;

[0031]FIG. 6 shows a timing diagram of monitor and purge operations ofthe system of FIG. 1 in accordance with a preferred embodiment of thepresent invention;

[0032]FIG. 7 shows a flow chart of a monitor-operation subprocess of theprocess of FIG. 5 in accordance with a preferred embodiment of thepresent invention;

[0033]FIG. 8 shows a side view of the filtration unit of FIG. 2 with thefilter cut away to demonstrate the monitor operation in accordance witha preferred embodiment of the present invention;

[0034]FIG. 9 shows a flow chart of a purge-operation subprocess of theprocess of FIG. 5 in accordance with a preferred embodiment of thepresent invention;

[0035]FIG. 10 shows a side view of the filtration unit of FIG. 2 withthe filter cut away to demonstrate the purge operation during an outwardstroke of the purge piston in accordance with a preferred embodiment ofthe present invention; and

[0036]FIG. 11 shows a side view of the filtration unit of FIG. 2 withthe filter cut away to demonstrate the purge operation during an inwardstroke of the purge piston in accordance with a preferred embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] In accordance with a preferred embodiment of the presentinvention, FIG. 1 shows a schematic view of a slurry monitoring system20, and FIGS. 2 and 3 show a side view and an end view, respectively, ofa filtration unit 22 for system 20. The following discussion refers toFIGS. 1, 2, and 3.

[0038] In the preferred embodiment, slurry monitoring system 20 isrepresented as monitoring the density of a lime-based slurry for use inscrubbing the exhaust of a coal-fired facility. Those skilled in the artwill appreciate that the present invention may be used to monitor anydesired characteristic of any slurry. The use of system 20 to monitorother than the density of a slurry, and/or the use of that slurry forother than the scrubbing of the exhaust of a coal-fired facility, doesnot depart from the spirit of the present invention.

[0039]FIG. 1 shows the basic components of slurry monitoring system 20.System 20 is made up of filtration unit 22 mounted within a slurry flowchamber 24. In the preferred embodiment, a mounting plate 26 is coupledto an opening in a wall 28 of flow chamber 24. Similarly, a base plate30 of filtration unit 22 is coupled to an opening in mounting plate 26.Auxiliary components (discussed hereinafter) are also mounted tomounting plate 26 and/or base plate 30 using conventional techniques(not shown). In this manner, filtration unit 22 and auxiliary componentsmay be removed from flow chamber 24 as a unit.

[0040] Those skilled in the art will appreciate that the method ofmounting filtration unit 22 and auxiliary components to flow chamber 24is not a part of the present invention. Other mounting methods may beused without departing from the spirit of the present invention.

[0041] Slurry flow chamber 24 may be of any desired form. In thepreferred embodiment, flow chamber 24 is a portion of a conduit for aslurry 32, e.g., a 24″ pipe.

[0042] Base plate 30 has a front side 33 and a back side 35 (FIG. 2). Inthe preferred embodiment, filtration unit 22 is made up of a filter 34enclosing a filter chamber 36. Filter 34 is formed of a screen 38coupled to front side 33 of base plate 34 at an inner end 40 of filterchamber 36, and an end plate 42 coupled to screen 38 at an outer end 44of filter chamber 36. End plate 42 is pierced by a plurality of holes43.

[0043] In the preferred embodiment, screen 38 is a woven metallic mesh.Those skilled in the art, however, will appreciate that this is not arequirement of the present invention. Screen 38 may assume other forms,e.g., a perforated metal sheet, without departing from the spirit of thepresent invention.

[0044] Slurry 32 is contaminated through use. That is, slurry 32contains components other than the liquid and suspended solid mattermaking up slurry 32 in its unused condition. In the preferredembodiment, slurry 32 is a lime-based slurry used to scrub the exhaustof a coal-fired facility. This use results in slurry 32 being acidic,abrasive, and lumpy. Slurry 32 may contain not only water and calciumoxide (lime) from the original mix, but also sulfur dioxide, hydrogenchloride (hydrochloric acid), fly ash, sulfuric acid, calcium sulfite,calcium sulfate dihydrate (gypsum), calcium chloride, agglutinatedsiliceous compounds, pieces of concrete, fiberglass, metal fragments,rebar, and chevrons.

[0045] It is desirable that filter 34 be protected from slurry 32 asmuch as is practical. Slurry 32 is acidic, abrasive, and lumpy. Filter34 is desirably protected against the acidity of slurry 32 through theuse of acid-resistant materials. Filter 34 is desirably protectedagainst the abrasiveness of slurry 32 through the use of materials ofsufficient hardness and/or resilience to resist abrasion. Filter 34 isprotected against the “lumpiness” of slurry 32 (i.e., against debris 46within slurry 32) by debris shield 48.

[0046] Debris 46 can assume significant size and mass. Debris shield 48,therefore, is desirably positioned and of sufficient strength to wardoff and protect filter 34 from direct impact by any debris 46 of a sizeand mass dangerous to filter 34. In the preferred embodiment, debrisshield 48 is substantially a pair of metal plates forming an inverted“V” over filter 34. One end of debris shield 48 is affixed to base plate30 in a conventional manner (e.g., by welding), and the other end ofdebris shield 48 is affixed to end plate 42 via a support member 50.Those skilled in the art will appreciate that the specific material,shape, and/or method of attachment of debris shield 48 not germane, andthat differing materials, shapes, and attachment methods may be employedwithout departing from the spirit of the present invention.

[0047] Filtration unit 22 includes an inlet pipe 52 containing a flushvalve 54 and, optionally, a block valve 55. Inlet pipe 52 is configuredto convey water 56 from a water source 58, through and under control offlush valve 54, through base plate 30, and into filter chamber 36.

[0048] Optional block valve 55 allows inlet pipe 52 to be closed forsafety during maintenance and/or replacement of filtration unit 22.Those skilled in the art will appreciate that optional block valve 55 isnot a requirement of the present invention, and is included as a safetymeasure in keeping with sound plumbing practices. The inclusion orexclusion of block valve 55 does not depart from the spirit of thepresent invention.

[0049] Filtration unit 22 also includes an outlet pipe 60 containing amonitoring device 62. Outlet pipe 60 is configured to convey a portionof slurry 32 from filter chamber 36, through base plate 30, throughmonitoring device 62, and to a sump 64. A desired characteristic ofslurry 32 is monitored as slurry 32 passes through monitoring device 62.

[0050] In the preferred embodiment, monitoring device 62 is configuredto monitor a density of slurry 32. Those skilled in the art willappreciate that this is not a limitation of the present invention.Monitoring device 62 may be configured to monitor another characteristicof slurry 32 (e.g., pH, opacity, etc.) without departing from the spiritof the present invention.

[0051] Within filter chamber 36, filtration unit 22 includes purgepiston 66. In the preferred embodiment, purge piston 66 is coupled to abidirectional ram 68 via a shaft 70, which passes through base plate 30.Purge piston 66 is therefore coupled to shaft 70 proximate front side 33of base plate 30, and bidirectional ram 68 is coupled to shaft 70proximate back side 35 of base plate 30.

[0052] Purge piston 66 is configured to reside in a retracted position72 proximate inner end 40 of filter chamber 36 a predetermined distance74 from base plate 30 during a monitor operation (discussedhereinafter), and to extend to, reside in, and retract from an extendedposition 76 position proximate outer end 44 of filter chamber 36 duringa purge operation (discussed hereinafter).

[0053]FIG. 4 shows a cross-sectional view of purge piston 66 forfiltration unit 22 in accordance with a preferred embodiment of thepresent invention. The following discussion refers to FIGS. 1, 2, 3, and4.

[0054] Fly ash is a pozzolan, and therefore cementitious within alime-based slurry. Being cementitious, fly ash is both cohesive andadhesive. That is, fly ash tends not only to bond with itself to formash masses, but also to cling to and coat the materials with which itcomes in contact. Fly ash, therefore, tends to cling to and clog filter34 and outlet pipe 60. This poses problems, which are addressedhereinafter in connection with the purge operation.

[0055] Fly ash also tends to cling to and coat purge piston 66. purgepiston 66, when in retracted position 72, is distance 74 away from baseplate 30 (discussed hereinafter). It is desirable that purge piston 66not be coated with fly ash, lest distance 74 be reduced. Purge piston 66is therefore desirably formed of (or has an outer surface 78 (FIG. 4)coated with) a non-metallic material 80 to which fly ash cannot easilyadhere. Desirably, non-metallic material 80 is a polymeric materialhaving a coefficient of friction of less than 0.3. In the preferredembodiment, purge piston 66 is formed of a single piece ofpolytetrafluoroethylene (PTFE).

[0056] Purge piston 66 may be fastened to shaft 70 in the mannerdemonstrated in FIG. 4. As depicted in FIG. 4, shaft 70 has a thread 82at one end thereof. Proximate thread 82, shaft 70 has affixed thereto ormachined therein a collar 84. Purge piston 66 is machined from a singlepiece of PTFE into a cylindrical disc having a front face 86, a backface 88, an edge surface 90, and a central hole 92 to receive shaft 70.Centered circular depressions 94 are machined into each of faces 86 and88 to receive force washers 96. Purge piston 66 and force washers 96 areplaced upon shaft 70 and seated at collar 84. Purge piston 66 and forcewashers 96 are secured in place by a shaft nut 98. Force washers 96 andshaft nut 98 may be formed of stainless steel or other suitablematerial, as may be shaft 70 itself.

[0057] Those skilled in the art will appreciate that other methods offabricating purge piston 66 and affixing purge piston 66 to shaft 70 maybe used without departing from the spirit of the present invention.

[0058]FIG. 5 shows a flow chart of a slurry monitoring process 200 foruse with system 20, and FIG. 6 shows a timing diagram of monitor andpurge operations 100 and 102 of system 20 in accordance with a preferredembodiment of the present invention. The following discussion refers toFIGS. 1, 2, 5, and 6.

[0059] In system 20, slurry monitoring process 200 provides for themonitoring of a characteristic of slurry 32 during monitor operation100, and for the purging of filter 34 and outlet pipe 60 during purgeoperation 102.

[0060] In a task 202, process 200 causes slurry 32 to flow through flowchamber 24 (FIG. 1). Task 202 may be effected by a pump (not shown), bygravity, or by other functions of scrubbing the exhaust of a coal-firedfacility.

[0061] Process 200 has two subprocesses, perform monitor operation 204and perform purge operation 206. In perform monitor-operation subprocess204 the desired characteristic of slurry 32 is monitored. In performpurge-operation subprocess 206, filter 34 and outlet pipe 60 are flushedand purged. Subprocesses 204 and 206 are repeated indefinitely in aclosed loop.

[0062] At any time during subprocess 204 a query task 208 may be used toinitiate perform purge operation 206 subprocess. Query task 208 may bein response to a manual action (e.g., the pressing of a button), timedaction (e.g., a timer closes a switch), or automatic action (in responseto a computer or other device). When query task 208 is affirmative,perform monitor operation subprocess 204 is terminated and controlimmediately passes to perform purge-operation subprocess 206.

[0063]FIG. 7 shows a flow chart of monitor-operation subprocess 204 ofprocess 200, and FIG. 8 shows a side view of filtration unit 22 withfilter 34 cut away to demonstrate monitor operation 100 in accordancewith a preferred embodiment of the present invention. The followingdiscussion refers to FIGS. 1, 5, 6, 7 and 8.

[0064] A pseudotask 210 of monitor-operation subprocess 204 maintainspurge piston 66 (FIG. 8) in retracted position 72 for a duration of amonitor time 104 (FIG. 6). Similarly, a pseudotask 212 maintains flushvalve 54 in closed condition 106 for the duration of monitor time 104.

[0065] In the preferred embodiment, pseudotasks 210 and 212 are effectedby refraining from doing a positive action. For example, prior topseudotask 210, purge piston 66 is in retracted position 72. Pseudotask210, therefore, consists of not doing anything to change the position ofpurge piston 66. Those skilled in the art will appreciate thatpseudotasks 210 and 212 may be active tasks, i.e., may require apositive action, in other embodiments (not shown). The use of such otherembodiments does not depart from the spirit of the present invention.

[0066] In a task 214, a portion 108 of slurry 32 is filtered byfiltration unit 22 to become filtered slurry 110. In task 214, slurry 32passes from flow chamber 24, through filter 34, and into filter chamber36.

[0067] Then in a task 216, a portion 112 of filtered slurry (F-SLURRY)110 is drawn into outlet pipe (O-PIPE) 60. This portion 112 of filteredslurry 110 passes through monitoring device 62 in a task 218, where thedesired characteristic of filtered slurry 110 is monitored.

[0068] Outlet pipe 60 has a predetermined inner diameter 114. Filter 34is desirably configured so that debris 46 within slurry 32 having adiameter 116 greater than one-half the predetermined outlet-pipe innerdiameter 114 is blocked. Filtered slurry 110, therefore, should containno debris 46 having a diameter 116 greater than one-half the innerdiameter 114 of outlet pipe 60. This significantly decreases thepossibility of having a debris jam within outlet pipe 60.

[0069] For this same reason, predetermined distance 74 between purgepiston 66 and base plate 30 is equal to or greater than inner diameter114 of outlet pipe 60.

[0070] Pseudotasks 210 and 212 are executed for the duration of monitortime 104. Tasks 214, 216, and 218 are performed substantiallycontinuously during pseudotasks 210 and 212, i.e., for the duration ofmonitor time 104. Monitor time 104 is significantly greater than purgetime 138 (see FIG. 6). The relationship between monitor and purge times104 and 138 is discussed in more detail hereinafter.

[0071] This completes monitor-operation subprocess 204, and control isreturned to process 200 (FIG. 1).

[0072]FIG. 9 shows a flow chart of purge-operation subprocess 206 ofprocess 200, and FIGS. 10 and 11 show a side view of filtration unit 22with filter 34 cut away to demonstrate purge operation 102 during anoutward stroke (FIG. 10) and an inward stroke (FIG. 11) of purge piston66 in accordance with a preferred embodiment of the present invention.The following discussion refers to FIGS. 1, 5, 6, 9, 10, and 11.

[0073] To better describe purge operation 102, filter chamber 36 may bethought of as being made up of two compartments. An inner compartment118 is that portion of filter chamber 36 between base plate 30 and purgepiston 66. An outer compartment 120 is that portion of filter chamber 36between purge piston 66 and end plate 42. The relative sizes of innerand outer compartments 118 and 120 therefore depends the position ofpurge piston 66 within filter chamber 36.

[0074] A task 220 of purge-operation subprocess 206 begins purgeoperation 102 by rapidly extending purge piston (P-PISTON) 66 fromretracted position 72 to extended position 76. This causes innercompartment 118 to rapidly increase in volume while causing outercompartment 120 to rapidly decrease in volume.

[0075] Substantially simultaneously, a task 222 opens flush valve 54,i.e., changes flush valve 54 from closed condition 106 (FIG. 8) to opencondition 122 (FIG. 10).

[0076] Those skilled in the art will appreciate that the order in whichtasks 220 and 222 are effected is not germane to the present invention.A reversal of the order of operations from that described herein doesnot depart from the spirit of the present invention.

[0077] A controller 124 is configured to cause bidirectional ram 68 totransit purge piston 66 between retracted and extended positions 72 and76. Controller 124 is also configured to cause flush valve 54 to assumeopen and closed conditions 102 and 106 (FIGS. 10 and 11). Tasks 220 and222 are accomplished in response to controller 124 (FIG. 1). Thoseskilled in the art will appreciate that controller 124 may beimplemented in any convenient manner of which those skilled in the artmay be cognizant. Typical implementations of controller 124 may includea simple manually operated switch, a timed switch, or a computer with asuitable program.

[0078] With flush valve 54 in open condition 122, a task 224 injectswater 56 into inner compartment (I-COMP) 118 of filter chamber 36. Thatis, water passes from water source 58, through inlet pipe 52, and intofilter chamber 36. Task 224 continues as long as flush valve 54 remainsin open condition 122.

[0079] Purge piston 66 has a thickness 126 equal to the distance betweenfront and back faces 86 and 88 (FIG. 4) proximate shaft 70, or proximateforce washers 96 if force washers 96 are used. Desirably, thickness 126is a maximum thickness 126, with the thickness of purge piston 66radially tapering to a minimum thickness 128 at edge surface 90. In thepreferred embodiment, this tapering is effected in part by a frontchamfer 130 (FIG. 4) between front face 86 and edge surface 90.Desirably, front chamfer 130 subtends an angle of 45° (±10°) relative tofront face 86.

[0080] As purge piston 66 transits from retracted position 72 toextended position 76 (FIG. 10), outer compartment 120 rapidly decreasesin volume. Compression effects increase the pressure of filtered slurry110 within outer compartment 120. This increase in pressure causes aportion 132 of filtered slurry 110 to be ejected from outer compartment(O-COMP) 120 through filter 34 and through holes #43 in end plate 42 ina task 226. This causes much of debris 46 clinging to filter 34 tobecome dislodged. Additionally, front chamfer 130 (FIG. 4) works withthe compression effects to accelerate and direct filtered slurry 110 outof outer compartment 120 in a shear field (not shown) to furtherdislodge debris 46.

[0081] In normal operation, a portion of filtered slurry 110 accumulatesin a lower portion of filter 34. Holes 43 in end plate 42 facilitate anin-line ejection of this accumulated filtered slurry 110 during task226.

[0082] Also, as purge piston 66 transits from retracted position 72 toextended position 76, inner compartment 118 rapidly increases in volume.Expansion effects decrease the pressure of filtered slurry 110 withininner compartment 118. This decrease in pressure causes a portion 134 offiltered slurry 110 within outlet pipe 60 to be extracted back intoinner compartment 118 of filter chamber 36 in a task 228. Debris 46 isthereby removed from outlet pipe 60. Desirably, outlet pipe 60 isconfigured to allow air 136 to enter outlet pipe 60, thereby permittingsubstantially all of filtered slurry 110 within outlet pipe 60 andmonitoring device 62 to be extracted back into inner compartment 118.

[0083] Following tasks 220 and 222, a pseudotask 230 of purge-operationsubprocess 206 retains purge piston 66 in retracted position 72 for aduration of a purge time 138 (FIG. 6). Similarly, a pseudotask 232retains flush valve 54 in open condition 122 for the duration of purgetime 138.

[0084] In the preferred embodiment, pseudotasks 230 and 232 are effectedby refraining from doing a positive action. For example, prior topseudotask 230, purge piston 66 is moved to extended position 72 by task220. Pseudotask 230, therefore, consists of not doing anything to changethe position of purge piston 66. Those skilled in the art willappreciate that pseudotasks 230 and 232 may be active tasks, i.e., mayrequire a positive action, in other embodiments (not shown). The use ofsuch other embodiments does not depart from the spirit of the presentinvention.

[0085] Flush valve 54 remains in open condition 122 for purge time 138,i.e., throughout pseudotasks 230 and 232. Water 56 continues to passfrom water source 58, through inlet pipe 52, and into inner compartment118 for the duration of purge time 138. This effects a task 234 todilute filtered slurry 110 in inner compartment 118 into diluted slurry140. At the end of purge time 138, diluted slurry 140 is desirablymostly water.

[0086] Monitor time 104 (see FIG. 6) is the time during which system 20monitors the desired characteristic of slurry 32. Purge time 138 issubstantially the time during which system 20 interrupts the monitoringof the desired characteristic of slurry 32 to purge filter 34, outletpipe 60, and monitoring device 62. Desirably, monitor time 104 issignificantly greater than purge time 138. In the preferred embodiment,i.e., where system 20 is used to monitor the density of a lime-basedslurry 32 used in the scrubbing of the exhaust of a coal-fired facility,a monitor time 104 of least 1 and no more than 48 hours with a purgetime 138 of at least 1.5 and no more than 5 seconds has been found to besatisfactory. Desirably, where controller 124 (FIG. 1) is automatic,monitor time 104 is 8 (±1) hours and purge time is 3 (+1, −0.5) seconds.

[0087] Following pseudotasks 230 and 232, a task 236 of purge-operationsubprocess 206 terminates (i.e., finishes) purge operation 102 byrapidly retracting purge piston 66 from extended position 76 toretracted position 72. This causes inner compartment 118 to rapidlydecrease in volume while causing outer compartment 120 to rapidlyincrease in volume.

[0088] Substantially simultaneously, a task 238 closes flush valve 54,i.e., changes flush valve 54 from open condition 122 to closed condition106. With flush valve 54 in closed condition 106, the flow of water 56from water source 58, through inlet pipe, and into filter chamber 36 iscurtailed.

[0089] As with tasks 220 and 222, tasks 236 and 238 are accomplished inresponse to controller 124. Those skilled in the art will appreciatethat the order in which tasks 236 and 238 are effected is not germane tothe present invention. A reversal of the order of operations from thatdescribed herein does not depart from the spirit of the presentinvention.

[0090] In the preferred embodiment, purge piston 66 has a back chamfer142 (FIG. 4) in addition to front chamfer 130 (FIG. 4). Back chamfer 142exists between back face 88 (FIG. 4) and edge surface 90. Desirably,back chamfer 142 subtends an angle of 450 (±10°) relative to back face88.

[0091] As purge piston 66 transits from extended position 76 toretracted position 72, inner compartment 118 rapidly decreases involume. Compression effects increase the pressure of diluted slurry 140within inner compartment 118. This increase in pressure causes a portion144 of diluted slurry (D-SLURRY) 140 to be flushed from innercompartment (I-COMP) 118 through filter 34 in a task 240. This causesfurther debris 46 clinging to the outside of filter 34 to becomedislodged. Additionally, back chamfer 142 (FIG. 4) works with thecompression effects to accelerate and direct diluted slurry 140 out ofinner compartment 118 in a shear field (not shown) to further dislodgedebris 46.

[0092] Also, the increase in pressure within inner compartment 118causes a portion 146 of diluted slurry 140 within inner compartment 118to pass through outlet pipe 60 and monitoring device 62 in a task 242.Task 242 substantially flushes and purges outlet pipe 60 and monitoringdevice 62 thereby.

[0093] Additionally, as purge piston 66 transits from extended position76 to retracted position 72, outer compartment 120 rapidly increases involume. Expansion effects decrease the pressure within outer compartment120. This decrease in pressure causes a portion 148 of slurry 110 withinflow chamber 24 to be drawn into outer compartment 120 of filter chamber36 in a task 244. Task 244 therefor fills outer compartment 120 withfiltered slurry 110 preparatory to returning to monitor operation 100.

[0094] In summary, the present invention teaches a slurry monitoringsystem 20 and a process 200 therefor. System 20 incorporates both amonitor operation 100 and a purge operation 102 in process 200. Duringmonitor operation 100 of system 20, process 200 uses a monitoring device62 to monitor the density of a slurry 32 through a filter chamber 36encompassed by a protected filter 34. During purge operation 102 ofsystem 20, process 200 purges filter chamber 36, filter 34, andmonitoring device 62.

[0095] Although the preferred embodiments of the invention have beenillustrated and described in detail, it will be readily apparent tothose skilled in the art that various modifications may be made thereinwithout departing from the spirit of the invention or from the scope ofthe appended claims.

What is claimed is:
 1. A slurry monitoring system configured to monitora characteristic of a slurry, said system comprising: a base plate; afilter coupled to said base plate; a filter chamber encompassed by saidfilter and configured to contain a filtered slurry; a purge pistoncontained within said filter chamber; an outlet pipe coupled to saidfilter chamber and configured to pass said filtered slurry; and amonitoring device coupled into said outlet pipe and configured tomonitor said characteristic of said filtered slurry.
 2. A slurrymonitoring system as claimed in claim 1 wherein said monitoring deviceis configured to monitor a density of said filtered slurry.
 3. A slurrymonitoring system as claimed in claim 1 wherein said slurry isconfigured to scrub an exhaust of a coal-fired facility, wherein saidslurry contains debris in the form of large and small ash particles,abraded cladding, and other materials, and wherein said systemadditionally comprises a debris shield configured to protect said filterfrom said debris.
 4. A slurry monitoring system as claimed in claim 1wherein said filter comprises a screen.
 5. A slurry monitoring system asclaimed in claim 4 wherein said screen is a wire mesh.
 6. A slurrymonitoring system as claimed in claim 1 wherein: said outlet pipe has aninner diameter; and said filter is configured to block debris fromentering said filter chamber when said debris has a diameter greaterthan one-half of said inner diameter of said outlet pipe.
 7. A slurrymonitoring system as claimed in claim 1 wherein said filter comprises: ascreen encompassing said filter chamber and coupled to said base plateat a first end of said filter chamber; and an end plate coupled to saidscreen at a second end of said filter chamber.
 8. A slurry monitoringsystem as claimed in claim 7 wherein said end plate comprises aplurality of holes.
 9. A slurry monitoring system as claimed in claim 1wherein said purge piston comprises: a first face; a second faceopposing said first face; and an edge surface between said first andsecond faces, wherein one of said first and second faces tapers towardssaid edge surface.
 10. A slurry monitoring system as claimed in claim 1wherein said purge piston comprises: a first face; a second faceopposing said first face; an edge surface between said first and secondfaces; and a chamfer between one of said first and second faces and saidedge surface.
 11. A slurry monitoring system as claimed in claim 10wherein said chamfer is substantially 45°±10°.
 12. A slurry monitoringsystem as claimed in claim 1 wherein said purge piston is formed of anonmetallic material.
 13. A slurry monitoring system as claimed in claim12 wherein said nonmetallic material is a polymeric material having acoefficient of friction of less than 0.3.
 14. A slurry monitoring systemas claimed in claim 12 wherein said nonmetallic material ispolytetrafluoroethylene.
 15. A slurry monitoring system as claimed inclaim 1 wherein said purge piston has an outer surface formed of anonmetallic material.
 16. A slurry monitoring system as claimed in claim1 additionally comprising: a shaft coupled to said purge piston withinsaid filter chamber; and a bidirectional ram coupled to said shaftwithout said filter chamber.
 17. A slurry monitoring system as claimedin claim 16 wherein: said filter chamber has a first end proximate tosaid base plate, and has a second end; said bidirectional ram isconfigured to cause said purge piston to reside in a first positionproximate said first end; and said bidirectional ram is additionallyconfigured to cause said purge piston to reside in a second positionproximate said second end.
 18. A slurry monitoring system as claimed inclaim 17 additionally comprising a controller configured to cause saidbidirectional ram to transit said purge piston between said first andsecond positions.
 19. A slurry monitoring system as claimed in claim 1additionally comprising: an inlet pipe coupled to said filter chamber;and a flush valve coupled into said inlet pipe and configured to controla flow of water into said filter chamber.
 20. A slurry monitoring systemas claimed in claim 19 additionally comprising a controller configuredto cause said flush valve to assume one of an open condition and aclosed condition.
 21. A slurry monitoring system as claimed in claim 19wherein said inlet pipe is coupled to said filter chamber through saidbase plate.
 22. A slurry monitoring method comprising: causing a slurryto flow through a slurry flow chamber into which a filter encompassing afilter chamber has been placed; maintaining a purge piston in a firstposition proximate a first end of said filter chamber for a firstpredetermined length of time to effect a monitor operation; extendingsaid purge piston from said first position to a second positionproximate a second end of said filter chamber to begin a purgeoperation; retaining said purge piston in said second position for asecond predetermined length of time to effect said purge operation; andretracting said purge piston from said second position to said firstposition to terminate said purge operation.
 23. A slurry monitoringmethod as claimed in claim 22 additionally comprising repeating saidmaintaining, extending, retaining, and retracting activities.
 24. Aslurry monitoring method as claimed in claim 22 additionally comprising:filtering a portion of said slurry from said slurry flow chamber throughsaid filter to produce a filtered slurry within said filter chamber;drawing a portion of said filtered slurry into an outlet pipe; andpassing said portion of said filtered slurry through a monitoring deviceto monitor a characteristic of said filtered slurry.
 25. A slurrymonitoring method as claimed in claim 22 additionally comprising:ejecting said filtered slurry from said filter chamber during saidextending activity; and extracting said filtered slurry from an outletpipe.
 26. A slurry monitoring method as claimed in claim 25 wherein saidsecond end of said filter chamber is closed by an end plate pierced by aplurality of holes, and wherein said ejecting activity ejects saidfiltered slurry from said filter chamber through said filter and throughsaid holes in said end plate.
 27. A slurry monitoring method as claimedin claim 25 additionally comprising: injecting water into said filterchamber from an inlet pipe; diluting said filtered slurry within saidfilter chamber with said water during said retaining activity to producea diluted slurry; and flushing said diluted slurry from said filterchamber during said retracting activity.
 28. A slurry monitoring methodas claimed in claim 27 additionally comprising purging said outlet pipeand a monitoring device coupled thereinto.
 29. A slurry monitoringmethod as claimed in claim 27 additionally comprising: opening a flushvalve to allow water to flow through an inlet pipe prior to saidinjecting activity; and closing said flush valve to curtail flow of saidwater prior to said flushing activity.
 30. A slurry-density monitoringsystem configured to monitor a density of a slurry in a slurry flowchamber during a monitor operation, and configured to self purge duringa purge operation, said system comprising: a base plate having a firstside and a second side; a filter having a first end and a second end,wherein said first end is coupled to said base plate upon said firstside; an end plate coupled to said second end; a filter chamberencompassed by said filter; a purge piston configured to reside withinsaid filter chamber in one of a first position proximate said first endand a second position proximate said second end; a shaft coupled to saidpurge piston proximate said first side; a bidirectional ram coupled tosaid shaft proximate said second side; an inlet pipe coupled to saidfilter chamber; a flush valve coupled into said inlet pipe; a controllerelectrically coupled to said bidirectional ram and said flush valve; anoutlet pipe coupled to said filter chamber; and a monitoring devicecoupled into said outlet pipe.
 31. A slurry-density monitoring system asclaimed in claim 30 wherein, during said monitor operation: said flushvalve maintains a closed condition; said bidirectional ram maintainssaid purge piston in said first position; a portion of said slurry insaid slurry flow chamber passes through said filter to become a filteredslurry within said filter chamber; a portion of said filtered slurrypasses into said outlet pipe; and said monitoring device monitors saidportion of said filtered slurry to determine a density thereof.
 32. Aslurry-density monitoring system as claimed in claim 30 wherein saidfilter chamber comprises: an inner compartment, being that portion ofsaid filter chamber between said purge piston and said base plate; andan outer compartment, being that portion of said filter chamber betweensaid purge piston and said end plate.
 33. A slurry-density monitoringsystem as claimed in claim 32 wherein, during said purge operation: saidcontroller instructs said flush valve to assume an open condition; saidinlet pipe passes water from a water source into said inner compartment;said controller instructs said bidirectional ram to transit said purgepiston from said first position to said second position; and said purgepiston forces a portion of said filtered slurry within said outercompartment to pass though said filter and into said slurry flow chamberduring transit of said purge piston.
 34. A slurry-density monitoringsystem as claimed in claim 33 wherein said portion of said filteredslurry forced to pass through said filter is a majority of said filteredslurry within said outer compartment.
 35. A slurry-density monitoringsystem as claimed in claim 33 wherein, during said purge operation, saidpurge piston forces a portion of said filtered slurry within said outletpipe to pass into said inner compartment during transit of said purgepiston.
 36. A slurry-density monitoring system as claimed in claim 32wherein, during said purge operation: said flush valve maintains an opencondition; said bidirectional ram maintains said purge piston in saidsecond position; said inlet pipe passes water from a water source intosaid inner compartment; and said water dilutes said filtered slurrywithin said inner compartment.
 37. A slurry-density monitoring system asclaimed in claim 32 wherein, during said purge operation: saidcontroller instructs said flush valve to assume a closed condition; saidcontroller instructs said bidirectional ram to transit said purge pistonwithin said filter chamber from said second position to said firstposition; said purge piston forces a portion of a diluted slurry withinsaid inner compartment to pass though said filter and into said slurryflow chamber during transit of said purge piston; and a portion of saidslurry in said slurry flow chamber passes through said filter to becomea filtered slurry within said outer compartment.
 38. A slurry-densitymonitoring system as claimed in claim 37 wherein said portion of saiddiluted slurry is a first portion, and wherein, during said purgeoperation: said purge piston forces a second portion of said dilutedslurry to pass into said outlet pipe during transit of said purgepiston; and said second portion of said diluted slurry purges saidoutlet pipe and said monitoring device.
 39. A slurry-density monitoringsystem as claimed in claim 30 wherein: said controller determines amonitor time, being a period of time during which said system effectssaid monitor operation between repeated ones of said purge operations;said controller determines a purge time, being a period of time saidsystem effects each of said purge operations; and said monitor time isgreater than said purge time.
 40. A slurry-density monitoring system asclaimed in claim 39 wherein: said monitor time is at least 1 and no morethan 48 hours; and said purge time is at least 1.5 and no more than 5seconds.